1. Field of the Invention
This invention relates to electronic article surveillance (EAS) systems, and more particularly to controlling the output power of an EAS transmitter using target range in an EAS interrogation zone.
2. Description of the Related Art
EAS systems are well known and are primarily used as a theft deterrent in retail establishments. U.S. Pat. No. 4,510,489 discloses one example of an EAS system that utilizes a marker adapted to resonate at a particular frequency provided by an incident magnetic field applied in an interrogation zone. One or more interrogation coils or antennas transmit the magnetic field, which defines the interrogation zone. Typically, antennas will be positioned at a store's exits to provide an interrogation zone through which customers must pass to exit the store. An active marker resonating in an interrogation zone is detected by EAS receive antennas and electronics, which can then trigger an alarm and/or result in other appropriate action. EAS systems detect the presence of an active marker anywhere in the interrogation zone. It would be advantageous, especially in applications involving very wide exits of 6 feet or wider, to determine where in the interrogation zone an active marker is located. The location of an active marker can aide in the identification of a potential shoplifter.
Presently, EAS interrogation antennas transmit at full power at all times to determine the presence of a marker. When an EAS marker is close to an antenna, full power is not necessary for detection, and needlessly causes excess power consumption. Constant operation at full power can also serve to reduce the long-term reliability of system components, causing increased service calls and failure rates. A marker placed outside, but close to the interrogation zone can, in certain circumstances, cause unintended alarms. An unintended alarm is an alarm that is due to the unintended detection of an active marker. Store personnel often display merchandise, with EAS markers attached, near store exits in the fringes of the intended interrogation zone that can sometimes cause unintended detection of the attached markers. The proximity of the EAS markers to the intended interrogation zone may cause an increased incidence of unintended alarms. Unintended alarms can result in an increased number of service calls, which unnecessarily increases the overall system operating expense. Detection of an active marker combined with detection of a target in the interrogation zone could eliminate the incidence of unintended alarms caused by markers being detected in areas adjacent to the intended interrogation zone. "Target" as used herein refers to people or other moving objects such as shopping carts capable of transporting an EAS marker into an interrogation zone.
In an attempt to solve some of the above mentioned problems, infrared beams and passive infrared (PIR) motion detectors have been used to detect people or other moving targets in the interrogation zone. In operation, if a marker is detected and there was no motion in the interrogation zone, then the detection was probably unintended. However, PIR detection zones often extended beyond the interrogation zone and result in detected motion when no one was actually in the interrogation zone. To try and control the PIR detection zone, freznel lenses were utilized that were difficult to set and control resulting in an expensive and less than ideal solution. Infrared detection of targets does not provide the capability, other than on/off control, of controlling transmitter power levels because only the presence or lack of presence of a target is detected. When transmitted, the interrogation electromagnetic field of present EAS systems is transmitted at full power.
What is needed is a solution to the problems discussed hereinabove, which includes transmitter power level control resulting in reduced incidence of unintended alarms, improved reliability, and reduced system operating and service costs.